The invention relates to detectors of ionizing radiation which use a photoconductor as a detector element.
Ionizing radiation detectors are used in numerous fields in order to provide an image of an object. Thus, for medical diagnosis, the body of a patient, or a portion thereof is irradiated with X-rays and the non-absorbed radiation is detected in order to make an image of the patient's skeleton.
In industrial testing, mechanical parts which are opaque to visible light are subjected to gamma rays or to X-rays having energy adapted to the absorption of the part under test, and the resulting image can be used for detecting faults such as microfractures.
In radiotherapy, the portion of the body to be treated is irradiated by beta or gamma radiation and non-absorbed radiation is used for forming an image of the treated portion and of its environment. This makes it possible to verify the patient's position and to change it if necessary so that the radiation does indeed reach the portion of the body to be treated, and that portion only.
The detectors in widest use are the following: a silver-based film which is generally used in association with a screen that transforms the ionizing radiation into radiation that marks the film; an X-ray image intensifier which is constituted by a vacuum tube containing a screen which transforms the ionizing radiation into light radiation, the tube also contains a photocathode which transforms the light radiation into electrons, and electron optics means for forming an electron image of the cathode on a screen which in turn transforms it into a light image; and a photoconductive detector which transforms the image formed by the ionizing radiation into an image constituted by a distribution of electric charge, with the commonest detector of this type being a xerographic plate where the photoconductor is selenium and where the electric charge distribution image is displayed by attracting an opaque powder (called "toner") onto a suitable medium.
Another photoconductive detector is described in the article entitled: "A liquid ionization detector for digital radiography of therapeutic megavoltage photon beams" by H. Meertens, et al, published in the Journal Phys. Med. Biol., 1985-Vol. 30, No. 4, at pages 313 to 321. As shown in FIG. 1, this detector comprises two grids 10 and 11 of linear electrodes 12 and 13 disposed in parallel planes. The electrodes within each grid are parallel to each other and perpendicular to the electrodes in the other grid. A photoconductive material (not shown) such as 2,2,4-trimethylpentane is disposed between the two parallel planes.
The electrodes 12 of the first grid 10 are connected to a biasing circuit 14 constituted by a voltage source 15 and a sequencer circuit 16. This circuit enables each of the electrodes in the first grid 10 to be biased relative to the electrodes 13 in the second grid 11 in such a manner that only one electrode is biased at a time, with the others remaining at the same potential as the second grid.
Each of the electrodes 13 in the second grid 11 is connected to an electrical charge measuring circuit 17 which memorizes its measurement. The read circuit 17 is essentially constituted by a current amplifier including a feedback capacitor 92. A sequencer circuit 18 serves to interrogate the measuring circuits 17 sequentially. In FIG. 1, arrow 19 indicates the propagation direction of the ionizing radiation.
The operation of such a photoconductive detector is now explained with reference to FIGS. 2a to 2e which are signal waveform diagrams as a function of time t. In these figures, the index x represents a magnitude relating to an electrode in the first grid 10 at position x in the X-Y co-ordinate system of FIG. 1. Similarly, the index y represents a magnitude relating to an electrode in the second grid 11, at position y in the X-Y co-ordinate system.
FIG. 2a shows the bias voltage as applied to the electrode at position x as a function of time.
FIG. 2b shows the bias voltage V.sub.x+1 as applied to the adjacent electrode at position x+1, similarly as a function of time.
FIG. 2c shows the electric charge density N.sub.x,y as a function of time at points in the photoconductor which are at the position x,y when projected onto the plane of the X-Y co- ordinate system.
FIG. 2d shows the electric charge density N.sub.x+1,y as a function of time for points of the photoconductor at the position x+1,y when projected onto the plane of the X-Y co-ordinate system.
Finally, FIG. 2e shows the current I.sub.y as measured on the electrode y in the second grid 11, as a function of time.
These various signal waveforms show that, at the instant when the electrode x in the first grid 10 is biased, the current I.sub.y measured on electrode x in the second grid 11 is proportional to the charge density N.sub.x,y at the cross-point between these electrodes: i.e. where the electrode x in the second grid 11 crosses the electrode x in the first grid 10.
In such a detector, the only radiation contributing to forming an image is the radiation received by that one of the electrodes in the first grid 10 which is polarized at a given instant, and this constitutes a low utilization level of the ionizing radiation. As a result image formation time is long and the object is subjected to a large amount of radiation.